Time-orthogonal CDMA wireless communication system

ABSTRACT

The system provides a slot-synchronized reverse link and transmission scheme where high-speed data transmissions are made possible by carrying the high-speed data transmission in slots orthogonal to the slots carrying control, voice and other low rate data transmission.

BACKGROUND OF THE INVENTION

[0001] In the evolving wireless data systems, such as the well-known1x-EV-DO and 1xEV-DV standards as well as the High Speed Downlink PacketAccess (HSDPA) specification in the Universal Mobile TelecommunicationSystem (UMTS) standard, the forward link (base station to mobilestation(s)) capacity has been increased by using techniques such as fastscheduling, adaptive modulation and coding (AMC) and hybrid ARQ (HARQ).In general, a scheduler, for example in the base station, selects a userfor transmission at a given time and adaptive modulation and codingallows selection of the appropriate transport format (modulation andcoding) for the current channel conditions seen by the user. Due toerrors in channel quality estimates, high error rates result in thetransmissions performed at a given rate (transport format). Hybrid ARQ,which makes use of fast retransmissions and combining a newly receivedcopy of the transmission with the previously received copies, allows forrecovery from transmission errors.

[0002] Further evolution of 3G standards includes high-speed reverselink packet access (mobile station to base station). The reverse link isdifferent from the forward link in the sense that the transmissions fromdifferent users are not orthogonal. In a CDMA system, both the forwardlink and the reverse link use orthogonal Walsh codes for spreading.Since the signal is transmitted from a fixed location (base station) onthe forward link, the different Walsh codes are still orthogonal whenthey arrive at the receiver. This is not the case on the reverse linkdue to the fact that the propagation times from mobiles at differentlocations to the base station are different. Therefore, orthogonalitycannot be guaranteed for signals coming from different mobiles. However,transmissions from the same mobile on different channels can still beorthogonal. The transmissions from multiple users interfere with eachother contributing to the noise rise seen by each of the users. Ingeneral, the noise rise at the base station is kept below a certainthreshold called the rise-over-thermal (RoT) threshold in order toguarantee desirable capacity and coverage. The circuitry of the basestation generates a certain amount of temperature dependent noise calledthermal noise. The RoT threshold limits the amount of power above thethermal noise at which mobiles can transmit. This limits the achievabledata rates and capacity for high-speed packet transmissions on thereverse link.

SUMMARY OF THE INVENTION

[0003] The present invention provides a time-orthogonal reverse linkchannel (mobile station to base station) whereby some of the slotswithin a frame, referred as burst (B) slots herein, are used for highspeed data transmissions while the remaining slots, referred to as powercontrol (PC) slots herein, are used to carry low data rate transmissionsand physical layer control signaling according to any well-knownstandard.

[0004] The transmissions in power control slots carry low rate datatransmission and physical layer control signaling and are powercontrolled as in a conventional CDMA system. The Rise-over-Thermal (RoT)is kept below a RoT threshold in these power control slots in order toguarantee an acceptable capacity/coverage to the critical real-timetraffic such as voice and physical layer control signaling. The burstslots are used for high-speed data transmissions in a time-multiplexedfashion, i.e. only one user transmits within a burst slot. Thetransmission within a burst slot is in one exemplary embodimentperformed at a mobile's peak power, but as described in detail below,the transmission within a burst slot is controllable and can be at somefraction of the mobile's peak power. Furthermore, the arrangement ofburst slots and power control slots in the reverse link channel iscontrollable, as is the data rate of transmission during a burst slot.

[0005] Since only a single user transmits during a burst slot, there isno interference from the users in the same cell. Therefore, no RoTconstraint needs to be respected i.e. the RoT during burst slots can bevery high with a large power from the user scheduled in the burst slot.Namely, a mobile transmitting during a burst slot will transmit at ahigher power than during a power control slot and will transmit at apower level regardless of the RoT threshold. This allows maximizing thereceived signal quality at the base station and therefore achieving veryhigh data rates.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, wherein like referencenumerals designate corresponding parts in the various drawings, andwherein:

[0007]FIG. 1 shows a reverse link channel having alternating powercontrol and burst slots according to an embodiment of the presentinvention;

[0008]FIG. 2 illustrates an example of low rate data transmission (e.g.,voice frames) in the power control slots of the reverse link channelshown in FIG. 1;

[0009]FIG. 3 illustrates an example of scheduling and encoder packettransmission according to an embodiment of the present invention;

[0010] FIGS. 4-6 illustrates additional exemplary embodiments ofscheduling and encoder packet transmission according to the presentinvention;

[0011]FIGS. 7 and 8 illustrate additional exemplary arrangements ofpower control and burst slots in frame; and

[0012] FIGS. 9-11 illustrate additional exemplary embodiments of encoderpacket transmission over one or more burst slots.

DETAILED DESCRIPTION

[0013] The present invention provides a time-orthogonal reverse linkchannel (mobile station to base station) whereby some of the slotswithin a frame, referred as burst (B) slots herein, are used for highspeed data transmissions while the remaining slots, referred to as powercontrol (PC) slots herein, are used to carry low data rate transmissionsand physical layer control signaling according to any well-knownstandard. The reverse link (RL) transmissions from different users areslot-synchronized. The slot synchronization is achieved by adjusting themobile transmit timing in such a way that the signals from multiplemobile stations arrive at the base station within some pre-specifieddelay threshold. For users in soft-handoff (SHO), the timing advance canbe adjusted to minimize the transmission overlap across the cells inSHO. The reception at different cells may also be synchronized (tocontrol inter-cell interference). In another embodiment of theinvention, the mobiles transmit timing are not adjusted. Therefore, somedegree of overlap depending upon the mobiles distance from the basestation may happen.

[0014] The invention will be described in the context of the 1xEV-DO(aka HRPD) system. However, the principles of the present invention canalso be applied to other CDMA systems such as cdma2000 (1xEV-DV), UMTSsystems, etc.

[0015] The physical layer signaling in 1xEV-DO, among others, consistsof pilot, channel quality feedback and ACK/NACK feedback for the HARQoperation as shown for one of the power control slots in FIG. 1. FIG. 1shows a reverse link channel having alternating power control and burstslots. The transmissions in power control slots carry low rate datatransmission and physical layer control signaling and are powercontrolled as in a conventional CDMA system. The Rise-over-Thermal (RoT)is kept below a RoT threshold in these PC slots in order to guarantee anacceptable capacity/coverage to the critical real-time traffic such asvoice and physical layer control signaling. The burst slots are used forhigh-speed data transmissions in a time-multiplexed fashion i.e. onlyone user transmits within a slot. The transmissions within a burst slotare in one exemplary embodiment performed at a mobile's peak power, butas described in detail below, the transmissions within a burst slot arecontrollable and can be at some fraction of the mobile's peak power.Since only a single user transmits during a burst slot, there is nointerference from the users in the same cell. Therefore, no RoTconstraint needs to be respected i.e. the RoT during burst slots can bevery high with a large power from the user scheduled in the burst slot.Namely, a mobile transmitting during a burst slot will transmit at ahigher power than during a power control slot and will transmit at apower level regardless of the RoT threshold. This allows maximizing thereceived signal quality at the base station and therefore achieving veryhigh data rates.

[0016] An example of data rates in burst slots is given in Table 1below. The encoder packet transmission duration is assumed to be 1 slot(1.67 ms). However, transmission duration of other than 1 slot can alsobe used to support high data rate transmissions in the burst slot. Forexample, the transmission duration can be fraction of a slot (e.g., ½ or¼ of a slot) or consist of several slots.

[0017] A time-multiplexed pilot is assumed in each burst slot. Notethat, as shown in FIG. 1, the pilot can also be code-multiplexed withinthe burst slot, where SB represents the burst signal power. The pilot inthe burst slot serves at least two purposes: (1) channel estimation fordemodulation and decoding of the traffic information, and (2) channelquality estimation of the current transmission. The channel qualityestimate can be used to select an appropriate modulation and codingscheme for any other new transmissions or retransmissions to the sameuser. The spreading is done by a single Walsh code in order to minimizethe impact on peak-to-average (PAR) requirements of the mobile stationpower amplifier. Note that at very high data rates, the spreading iszero. Multicode transmissions can also be considered if PAR requirementscan be met. TABLE 1 Data rates Encoder Num- Num- Debt packet SF ber ofber of T/P rate size Modu- Coding (Single data pilot ratio [Kb/s] [bits]lation rate code) chips chips [dB] 4003.2 6672 16-QAM 0.834 1 2000 4816.20 3686.4 6144 16-QAM 0.768 1 2000 48 16.20 3072.0 5120 16-QAM 0.6401 2000 48 16.20 2457.6 4096 8-PSK 0.688 1 1984 64 14.91 1843.2 30728-PSK 0.525 1 1952 96 13.08 1228.8 2048 QPSK 0.533 1 1920 128 11.76614.4 1024 QPSK 0.276 2 1856 192 9.85 307.2 512 QPSK 0.276 2 1856 1929.85 153.6 256 QPSK 0.276 4 1856 192 9.85 76.8 128 BPSK 0.276 4 1856 1929.85

[0018] An example, of low rate data transmission (e.g., voice frames) inthe PC slots is shown in FIG. 2. Note that multiple simultaneouscode-multiplexed transmissions can be performed in the PC slots.However, a single user transmits during a burst slot.

[0019] In operation, the base station generates and sends a schedulingor schedule grant message on the forward link. In one exemplaryembodiment, the scheduling grant message includes a scheduling grantmessage indicator to indicate that the message is a schedule grantmessage. In another embodiment, a forward link channel (e.g., an entirephysical channel or slots of a physical channel) is dedicated totransmission of schedule grant messages.

[0020] In one exemplary embodiment, the scheduling grant messageincludes one or more mobile identifiers. Associated with each mobileidentifier in the schedule grant message are a slot indicator, a datarate indicator and a power level indicator. The mobile identifier is anywell-known identifier for a base station to identify a mobile station.The slot indicator indicates the one or more slots in the reverse linkchannel over which the mobile station identified by the associatedmobile identifier should perform a burst slot transmission. The datarate indicator indicates the data rate of the burst slot transmission.The power level indicator indicates the power level that the mobilestation should transmit at during the burst slot transmission.

[0021] It will be understood that one or more of the schedule grantmessage parameters associated with a mobile identifier can be fixed fora given wireless communication system. This would then eliminate theneed for having this parameter in the schedule grant message. Forexample, if the power level for a given wireless communication systemwere fixed at peak power or some fraction thereof, then this wouldeliminate the need for the power level indicator in the schedule grantmessage. As another example, because of the known timing relationshipbetween the forward link and reverse link as well as the time needed forthe mobile station to generate an encoder packet, a standard can fixburst slot transmission by a mobile station identified by the mobilestation identifier in the reverse link channel slot following generationof the encoder slot in response to the schedule grant message. Thiswould eliminate the need to provide the slot indicator in the schedulegrant message. As a further example, the data rate can be fixed, or madedependent on the power level at which the mobile is able to transmitduring the burst slot. This would eliminate the need for the data rateindicator.

[0022] Mobile stations monitor the forward link and identify theschedule grant messages based on the message indicator or based on theforward link channel being monitored. For identified schedule grantmessages, mobile stations determine whether their mobile identifier iscontained in the schedule grant message. Further processing of theschedule grant message depends on the standardization of the schedulegrant message.

[0023] If the standard has not fixed the format of the reverse linkchannel including burst slots, then the schedule grant message willinclude one or more slot indicators in association with each mobileidentifier. In this embodiment, each mobile determines the burst slotsfrom the slot indicators regardless of whether their mobile identifieris included in the schedule grant message. In this manner, each mobileidentifies the burst slots, and for those burst slots in which themobile has not been scheduled to transmit, the mobile does not transmit.If the standard has fixed the format of the reverse link channelincluding burst slots, then mobile stations only need to initiallyexamine the schedule grant message for their identifiers. If a mobilestation does not find its identifier in the schedule grant message, nofurther processing of the schedule grant message is required. By notfinding its mobile identifier, a mobile station will remain silentduring the burst slots of the reverse link channel.

[0024] However, when a mobile station in either the fixed or flexibleburst slot embodiments identifies its mobile identifier in the schedulegrant message, the mobile station accesses the data rate indicator (ifpresent) and the power level indicator (if present) associated with itsmobile identifier in the schedule grant message. The mobile thengenerates an encoder packet in the well-known manner and transmits theencoder packet during the burst slot fixed by the standard or identifiedby the slot indicator. The mobile transmits the encoder packet at a datarate identified by the data rate indicator and at a power levelidentified by the power level indicator.

[0025] As will be appreciated, if a standard fixes the data rate, powerlevel and format of the reverse link channel, then mobiles will onlyexamine the schedule grant message for their mobile identifiers.

[0026] An example of scheduling and encoder packet transmission is shownin FIG. 3. In this example, the format, data rate and power level arefixed by the standard. The format is fixed to alternate power controland burst slots as shown in FIG. 1. As shown in FIG. 3, the base stationsends the schedule grant message over one slot (1.67 ms) that includes amobile station identifier. After some propagation delay (T_(p)), theidentified mobile station receives the grant message in slot#1. Inslot#2, after time T_(s) to process the scheduling grant message, themobile station identifies its identifier in the schedule grant message.In response to this identification, the mobile station during timeT_(ep) forms an encoder packet and transmits the encoder packet inslot#3. The base station always knows the identity of the mobile stationtransmitted during a particular slot due to the fixed timingrelationship between schedule grant message transmission by the basestation and encoder packet transmission by the mobile station (i.e. forevery scheduling grant message sent on the forward link in slot#i, theencoder packet is transmitted in slot#(i+2) on the reverse link).Therefore, the mobile station does not need to include its identifierwith encoder packet transmission.

[0027] Another example of encoder packet scheduling is given in FIG. 4.In this example, both the scheduling grant transmission duration andencoder packet transmission duration is one slot (1.67 ms). In thisexample, mobile station U1 transmits in slots 3, 7 and 9 on the reverselink (RL), mobile station U2 transmits in slots 5, 11, 13 and 15 andmobile station U3 transmits in slot 1.

[0028]FIG. 5 shows an example where the scheduling grant transmissionduration is 2 slots (3.33 ms) while the encoder packet duration is oneslot (1.67 ms). FIG. 6 shows an example where a single schedule grantmessage carries information for more than one encoder packettransmissions—the two encoder packet transmission being for differentmobiles.

[0029]FIGS. 7 through 8 illustrate some exemplary arrangements of thepower control and burst slots within a 26.67 ms frame. In FIG. 7 twoconsecutive burst slots are scheduled for each two consecutive powercontrol slots (a 2-2 configuration). In FIG. 8 three consecutive burstslots are scheduled for each power control slot (a 1-3 configuration).

[0030] It will be further appreciated that depending on the data rateand standard, one or more encoder packets or only a portion of anencoder packet can be transmitted during a burst slot. FIG. 9 shows thateach encoder packet (EP) is transmitted over a single burst slot of the1-3 configuration. FIG. 10 shows each encoder packet (EP) is transmittedover three consecutive burst slots of the 1-3 configuration, and FIG. 11shows an encoder packet (EP) is transmitted over four non-consecutiveburst slots in the 1-1 configuration of FIG. 1.

[0031] In cases where the burst mode of transmission according to thepresent invention has to coexist with simultaneous power control slottransmission, e.g., from legacy mobiles, a high-speed data user canstill be allowed to transmit at, for example, peak power during theburst slot. The increased interference in the power control slots of thelegacy mobiles can be compensated for to some extent by increasing theouter loop power control set point for those mobiles. Anotherpossibility is to employ interference cancellation in order to subtractthe burst user signal from the overall received signal. This way thestronger signal from the burst user can first be detected and subtractedform the overall signal reducing the impact of burst user on powercontrolled legacy transmissions.

[0032] The cancellation technique, in another exemplary embodiment ofthe present invention, can also be applied to allow a limited number ofnon-legacy mobiles to transmit during a burst slot.

[0033] The present invention provides a transmission scheme on aslot-synchronized reverse link of a CDMA wireless system where thehigh-speed data transmissions are carried in slots that are orthogonalto the slots carrying control, voice and other low rate datatransmission.

[0034] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations arc not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

We claim:
 1. A method of transmitting information over a reverse link bya mobile, comprising: transmitting at a higher power than thatestablished by a power control algorithm during a burst slot of areverse link channel.
 2. The method of claim 1, wherein the transmittingstep transmits at the higher power level in response to a schedule grantmessage.
 3. The method of claim 2, further comprising: receiving theschedule grant message, the schedule grant message including at leastone mobile identifier; and wherein the transmitting step transmits atthe higher power when the mobile identifier in the schedule grantmessage matches a mobile identifier of the mobile.
 4. The method ofclaim 3, further comprising: performing no transmission when theschedule grant message does not include a mobile identifier of themobile.
 5. The method of claim 3, further comprising: generating anencoder packet in response to the received schedule grant message whenthe mobile identifier in the schedule grant message matches a mobileidentifier of the mobile; and wherein the transmitting step transmitsthe encoder packet.
 6. The method of claim 3, wherein the schedule grantmessage includes a data rate; and the transmitting step transmits at thedata rate.
 7. The method of claim 3, wherein the schedule grant messageincludes a power level; and the transmitting step transmits at the powerlevel.
 8. The method of claim 3, wherein the schedule grant messageincludes a slot indicator associated with the mobile identifieridentifying a burst slot on the reverse link channel; and thetransmitting step transmits at the higher power level during theidentified burst slot when the mobile identifier in the schedule grantmessage matches a mobile identifier of the mobile.
 9. The method ofclaim 2, wherein the schedule grant message includes a power level; andthe transmitting step transmits at the power level.
 10. The method ofclaim 2, wherein the schedule grant message includes a slot indicatoridentifying a burst slot on the reverse link channel; and thetransmitting step transmits at the higher power level during theidentified burst slot.
 11. The method of claim 1, wherein the higherpower is a peak power of the mobile scheduled to transmit during theburst slot.
 12. The method of claim 1, wherein the higher power is afraction of the peak power of the mobile scheduled to transmit duringthe burst slot.
 13. The method of claim 1, further comprising:generating at least one encoder packet in response to a receivedschedule grant message; and wherein the transmitting step transmits atleast a portion of the encoder packet.
 14. The method of claim 13,wherein the transmitting step transmits one encoder packet over a singleburst slot.
 15. The method of claim 13, wherein the transmitting steptransmits one encoder packet over more than one burst slot.
 16. Themethod of claim 1, further comprising: transmitting at a powerestablished according to a power control algorithm during power controlslots of the reverse link channel.
 17. The method of claim 16, whereinthe power control slots are time orthogonal to the burst slots.
 18. Amethod of transmitting information over a reverse link by a mobile,comprising: transmitting at a power level during a burst slot of areverse link channel regardless of a RoT threshold.
 19. A method ofcontrolling transmission over reverse links by mobiles, comprising:transmitting a schedule grant message on a forward link channel, theschedule grant message identifying at least one mobile to transmit at ahigher power level than that established by a power control algorithmduring a burst slot of a reverse link channel.
 20. The method of claim19, wherein the schedule grant message includes at least one mobileidentifier identifying the mobile.
 21. The method of claim 19, whereinthe schedule grant message includes a data rate indicator indicating adata rate at which the mobile is to transmit during the burst slot. 22.The method of claim 19, wherein the schedule grant message includes apower level indicator indicating a power level at which the mobile is totransmit during the burst slot.
 23. The method of claim 19, wherein theschedule grant message includes a slot identifier identifig the burstslot in the reverse link channel over which the mobile is to transmit.24. The method of claim 19, wherein the schedule grant messageidentifies more than one mobile to transmit at a high power level, andidentifies different burst slots of the reverse link channel over whicheach mobile is to transmit.
 25. The method of claim 19, furthercomprising: scheduling which slots of the reverse link channel will beburst slots; and wherein the transmitting step transmits one or moreschedule grant messages based on the scheduling.
 26. The method of claim25, wherein the scheduling step schedules every other slot of thereverse link channel as a burst slot.
 27. The method of claim 25,wherein the scheduling step schedules two consecutive burst slots foreach two consecutive non-burst slots.
 28. The method of claim 25,wherein the scheduling step schedule three consecutive burst slots foreach non-burst slot.
 29. The method of claim 25, wherein the schedulingstep schedules which mobile will transmit during which burst slot. 30.The method of claim 25, further comprising: adjusting a threshold of thepower control algorithm such that at least one mobile unable to operateaccording to the schedule grant message increases transmission powerduring non-burst slots of the reverse link channel.
 31. The method ofclaim 19, further comprising: receiving an overall signal; detecting aburst transmission signal in the overall signal; and performinginterference cancellation on the overall signal using the detected bursttransmission signal.
 32. A method of controlling transmission overreverse links by mobiles, comprising: transmitting a schedule grantmessage on a forward link channel, the schedule grant messageidentifying at least one mobile to transmit during a burst slot of areverse link channel regardless of a RoT threshold.